The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) communication system and OFDM communication method, and more particularly to the base station, mobile stations and OFDM communication method of an OFDM communication system that divides a band region into a plurality of bands and assigns each band to a mobile station.
OFDMA Access Method
In cellular mobile communication that uses an OFDM communication method, an access method is known called OFDMA (Orthogonal Frequency Division Multiple Access) that multiplexes users by dividing a band region into a plurality of bands and respectively assigning each of the bands to each of a plurality of users. FIG. 18 is a drawing showing the state of user division in a frequency band region of the OFDMA access method. In (A) of FIG. 18 an example is shown in which a band region that comprises 31 subcarriers is divided into 3 bands having 10 subcarriers, 11 subcarriers and 10 subcarriers, and respectively assigns each of the bands to different users.
Applying OFDMA to a Downlink
The construction of a base station transmitter in the case of applying OFDMA to a downlink communication (communication from a base station to a mobile station) is shown in FIG. 19, and the construction of a mobile station receiver is shown in FIG. 20. In a downlink the transmission data for the three users that have been assigned to each of the bands is distributed to each of the subcarriers 1 to 31 shown in (B) of FIG. 18, and input to an IFFT unit 1. The IFFT unit 1 performs IFFT processing on the subcarrier signals and converts them to a time domain signal, and a guard interval insertion unit 2 inserts guard intervals (GI) into that time domain signal. Here, as shown in FIG. 21, a guard interval GI is created by copying the last portion of the OFDM symbol. The baseband signal in which GI have been inserted is converted to an analog signal by a DA converter 3a of a transmission circuit (Tx) 3, after which, a frequency conversion unit 3b converts the frequency to an RF signal having a center frequency f1, and a bandpass filter 3c restricts the band, after which the signal is amplified and transmitted from a transmission antenna 4. In (B) of FIG. 18, the bandwidths and center frequencies of each band in the RF signal after frequency conversion are shown, where a band region having bandwidth B1 (MHz) is divided into three bands of bandwidth B2 (MHz), with the center frequencies of each band being f0, f1 and f2.
Also, the base station uses the frame format shown in FIG. 22 to insert and transmit at fixed intervals well-known pilot signals that are necessary for channel estimation by a mobile station to perform channel estimation. The frame comprises N number of OFDM symbols, and a pilot symbol and control data symbol are inserted for each frame.
The signal that is output from the transmission antenna 4 is received by the reception antenna 5 (FIG. 20) of the mobile station by way of a fading propagation path, and a reception circuit (Rx) 6 converts the RF signal ((B) of FIG. 18) that is received by the antenna to a baseband signal. In other words, the bandpass filter 6a of the passed band region B1 restricts the band of the RF signal that is received by the antenna 5 and inputs the signal to a low-noise amp 6b, and the low-noise amp 6b amplifies the signal to specified power. A mixer 6c multiplies the output signal from the low-noise amp 6b by a local signal having the center frequency of the band to be demodulated, and converts the RF signal after power amplification to a baseband signal. For example, by assuming that band 2 is the object of demodulation by the mobile station, the local oscillator 6d generates a local signal having frequency f1, and the mixer 6c converts the signal to a baseband signal by multiplying that local signal by the RF signal. Here, an example of directly converting from an RF signal to a baseband signal was presented, however, there is also a method of dropping the frequency first to an intermediate frequency.
As shown in (B) of FIG. 18, the signal after baseband conversion is input to an AD converter 6f by way of an anti-aliasing low-pass filter 6e having the property A of a cutoff frequency B2/2 (MHz). An AD converter 6f uses a sampling rate that is double the bandwidth B2 to convert the signal to digital data. Finally, a FIR filter 6g, having a cutoff frequency B2/2 (MHz), extracts the signal of a desired band from the signal after AD conversion and outputs the result.
An FFT timing synchronization circuit 7 detects FFT timing from a time domain signal that includes a signal of a desired band that is output from the reception circuit 6, and a symbol extraction unit 8 extracts the symbol at that timing and inputs it to an FFT unit 9. The FFT unit 9 performs FFT processing for each extracted symbol, and converts the signal to a subcarrier signal in the frequency domain. By calculating the correlation between pilot symbols that are received at set intervals, and a well-known pilot pattern, a channel estimation circuit 10 performs channel estimation for each subcarrier, and a channel compensation circuit 11 uses the estimated channel value to compensate for channel fluctuation of the data symbol. Through the processing described above, the transmission data that is distributed to each of the subcarriers 1 to 31 shown in FIG. 18 is demodulated, however, it is also possible for an OFDMA receiver to perform demodulation of only the subcarrier signals of the band assigned to the local station. In the example shown in FIG. 20, The FFT unit 9 outputs subcarrier signals 11 to 21 of the band 2, and the channel compensation unit 11 performs channel compensation and outputs demodulated data. As shown in the frame format of FIG. 22, the mobile station is notified by way of the time-multiplexed control channel of the information of the band assigned to the local station. After that, the demodulated subcarrier signals 11 to 21 are converted to serial data, and then decoded.
Applying OFDMA to an Uplink
FIG. 23 is a drawing showing the construction of a mobile station when OFDMA is applied to an uplink communication (communication from a mobile station to a base station), and FIG. 24 is a drawing showing the construction of a base station.
As shown in (A) of FIG. 18, band 1 to band 3 are each assigned to different mobile stations 201 to 203. In each of the mobile stations 201 to 203, the transmission data from the users are input to IFFT units 211 to 213 as subcarrier signals 1 to 10, 11 to 21 and 22 to 31. The IFFT units 211 to 213 perform IFFT processing on the respective subcarrier signals, and convert the signals to time-domain signals, after which guard interval units 221 to 223 insert guard intervals GI into the time-domain signals. Transmission circuits (Tx) 231 to 233 convert the input signals to analog signals, and then converts the frequency to an RF signal having center frequencies f0 to f2 that correspond to each band, and after the bands are restricted, the signals are amplified and transmitted from transmission antennas 241 to 243.
The OFDM modulated signals that are transmitted from each of the mobile stations pass through respective propagation paths and are received by the reception antenna 31 (see FIG. 24) of the base station, then the reception circuit (Rx) 32 converts the RF signals to baseband signals. In other words, the bandpass filter 32a of the passing band region B1 restricts the band of the RF signal that is received by the antenna 31 and inputs the result to a low-noise amp 32b, and the low-noise amp 32b amplifies the power of the signal to a specified power. By multiplying the signal output from the low-noise amp 32b by the signal having the center frequency f1 of the band B1 that is output from a local oscillator 32d, a mixer 32c converts the power-amplified RF signal to a baseband signal. The signal after baseband conversion is input to an AD converter 32f via an anti-aliasing low-pass filter 32e having a cut-off frequency B1/2 (MHz). The AD converter 32f uses a sampling rate that is double the bandwidth B1 to convert the signal to digital data and output the result.
A FFT timing synchronization circuit 33 detects the FFT timing from the time domain signal that includes the signals of each of the bands that were output from the reception circuit 32, and at that time, a symbol removal unit 34 extracts symbols, and inputs them to a FFT unit 35. The FFT unit 35 performs FFT processing on each of the extracted symbols, and converts them to frequency domain subcarrier signals. By calculating the correlation between pilot symbols that are received at fixed intervals, and a well-known pilot pattern, a channel estimation circuit 36 performs channel estimation for each subcarrier, and a channel compensation circuit 37 uses the estimated channel values to compensate for channel fluctuation of data symbols. Through the processing described above, transmission data from three users that was distributed to all of the subcarriers 1 to 31 shown in (A) of FIG. 18 is demodulated. After that, the demodulated subcarrier signals 1 to 31 (not shown in the figure) are converted to serial data and decoded for each band.
Guard Band
When the OFDMA access method is applied to a downlink communication, after the band that is assigned to each of the mobile stations is selected by a reception filter (low pass filter 6e shown in FIG. 20) having characteristic A as shown in (A) of FIG. 25, a receiver (FFT, channel compensation unit, etc.) having a specified bandwidth performs reception processing. At this time, orthogonality between subcarriers is lost due to the waveforms of the subcarriers are distorted in a sloped area A′ of the band 2 which is restricted by the reception filter, and there is a problem in that the interference component is leaked into the regions of the bands.
Therefore, as shown in (B) of FIG. 25, the effect of the interference described above is removed by placing guard bands (subcarriers 10, 11; 21, 22) at the boundaries of the bands, and by not using the subcarriers of those areas for data transmission. A method for extracting band 2 with a reception filter at a mobile station is shown in (C) of FIG. 25. As shown in (C) of FIG. 25, by performing design so that sloped portion of the reception filter comes into the guard band region, it is possible to eliminate the effect of interference due to waveform distortion. In the case of using a reception filter having wide passband characteristic as shown in (A) of FIG. 25 as well, a comparatively large interference component is generated in the guard band region, so it is possible to prevent the effect of the interference component leaking into the band.
On the other hand, when OFDMA is applied to an uplink communication (communication from a mobile station to the base station), the base station receives a plurality of bands all together and performs OFDM signal processing. Generally, as shown in FIG. 21, in OFDM, by inserting a guard interval GI by copying the last part of the signal waveform and adding it to the start of the OFDM symbol, orthogonality between subcarriers can be maintained even when receiving signals such multipath signals or signals for other users having different reception timing. This will be simply explained using FIG. 26. The FFT timing synchronization unit 33 (see FIG. 24) of the base station measures the reception timing (FFT timing) of signals from a plurality of users that are received at the same time, and the symbol extraction unit 34 determines the symbol position based upon the earliest FFT timing and from the received signals extracts symbols from which the guard interval has been removed (the main wave of user 1 in the example shown in FIG. 26) and performs FFT processing. When doing this, in the case that signals from all users including guard intervals, are included in the extracted symbol, orthogonality between subcarriers can be maintained depending on the nature of FFT. However, in an uplink, the timing at which signals arrive at the base station differs greatly for each user due to the distance between the base station and mobile stations and the propagation state, so the difference in reception timing may exceed the guard interval, and thus a state occurs in which orthogonality between subcarriers is lost. In this case, by inserting guard bands as shown in (B) of FIG. 25, it is possible to reduce the effect of interference due to the loss of orthogonality between subcarriers of adjacent bands.
Also, in mobile communication, offset in carrier frequency occurs due to small shifts in the reference frequencies of the base station and mobile station. Normally, in a mobile station, the offset in carrier frequency is compensated by AFC (Automatic Frequency Control), however, since the performance of the AFC differs depending on the mobile station, the frequency offset that cannot be completed compensated for by AFC differs for each user. For example, it is known that when the amount of frequency offset reaches about 10% of the gap between subcarrier frequencies, the transfer properties greatly deteriorate due to the effect of interference between subcarriers. In this kind of case, by inserting guard bands as shown in (B) of FIG. 25, it is possible to reduce the effect of interference from bands assigned to users having poor AFC performance.